The present invention relates to an apparatus for ascertaining a predetermined rotary position of a rotational body, and in particular the supply onset position of the drive shaft of a fuel injection pump for an internal combustion engine.
For adjusting, testing and relocating a predetermined rotary position of a rotational body, it is necessary to be able to adjust precisely the rotary position of an angle indication marking firmly connected with the rotational body, or, following adjustment, to be able to perform precise measurement with reference to the housing of the rotational body. In particular in fuel injection pumps, it is necessary to measure the supply onset position or a rotary position corresponding to the supply onset of the cam, associated with the first pump outlet after the drive mechanism of the cam shaft, here representing the rotational body or drive shaft; subsequently, the cam shaft must be held in a fixed position in order to be able to mount the injection pump on the appropriately prepared internal combustion engine. Now in order to be able to adjust the supply onset position of the cam shaft initially, various static and also dynamic measurement procedures are known. The oldest static measurement procedure is the so-called overrun method, by means of which the static supply onset of the injection pump, which has already been adjusted in terms of the preliminary stroke, can be looked for. The pump section chamber placed under fuel pressure and the cam shaft is slowly rotated until the pump piston closes the suction bore during its upward stroke, and the fuel stops flowing out. In this rotary position, a marking for supply onset is made by means of a slash found on the housing; however, this marking is too imprecise for present-day stringent exhaust gas regulations, and it has the grave disadvantage that the slash marking either cannot be seen from outside or can be seen only with difficulty once the pump is mounted to the gear box of the engine, for instance with an end flange.
Dynamic measurement procedures, such as those known or proposed in German Offenlegungsschrift (laid application) 27 00 878 and German patent application P 29 49 018.4, are more precise but in terms of the expenditure for measurement compliance they are very expensive. However, such procedures can be used in the manufacturing operation for injection pumps for the initial establishment of the predetermined rotary position of the angle indication marking.